In current conventional drilling process, overbalanced drilling technology tends to be used, that is, the BHP during drilling operation (drilling fluid column pressure plus circulating pressure drop) is higher than formation pore pressure. The advantage of this technology is its high safety. However, because drilling fluid pressure is higher than formation pore pressure, formation pollution is inevitable, in that (1) mud filtrates invade into formation and are hydrated with clay in the formation, which results in clay swelling, dispersion and migration and plugging of pore throats; (2) the chemical reaction between mud filtrates and formation fluids leads to water blocking, emulsification, wettability reversal and solid precipitations resulting in plugging of pore throats; (3) solid precipitation from mud plugs pore throats directly. Due to the above reasons, in onsite drilling operation, although good oil and gas shows are observed before well completion and post-completion effect reaction is strong even with well kick and well blowout, the effect for well completion testing are very poor and production (if any) is rather low or declines rapidly owing to reservoir pollution and other reasons. In such case, the good oil and gas shows in drilling process make the decision makers reluctant to give up the opportunity, thus wells are drilled repeatedly, resulting in waste of huge investment, delay or even missing the discovery of new oilfields. Furthermore, the pressure difference can exert negative influence on penetration rate, such as (1) influence on rock strength: the bigger the pressure difference is, the higher the rock strength is and the harder to crash the rock; (2) influence on hole bottom cleaning: higher pressure difference tends to result in chip hold down effect and affects penetration rate, so the higher the pressure difference is, the lower the penetration rate is. Therefore, reducing pressure difference is one way to improve penetration rate.
As one of the top 10 leading petroleum-engineering technologies in the 20th century, underbalance drilling (UBD) has been experienced rapid development abroad as an emerging technology in recent years. It is designed to avoid those serious engineering accidents occurred in overbalanced drilling operation including lost of well, improve penetration rate and mitigate formation damage. It leads to breakthrough in well drilling theory and is the inevitable result of the transition of drilling operation from overbalanced drilling, balanced drilling to underbalance drilling.
UBD is characterized by the utilization of special equipment (rotary blowout preventer) and process to conduct underbalance drilling at borehole bottom, i.g. Drilling while jetting. The key point for UBD is to keep bottom hole pressure (BHP) lower than formation pore pressure or formation pressure within a proper range (i.e., set negative pressure value) during drilling operation. However, in actual drilling operation, BHP can never be kept constant as a result of the fluctuation of wellhead pressure and bottom hole pressure, mainly because formation fluid enters into the hole, especially formation gas flows into the wellbore under the negative pressure at hole bottom and pump-in flow rate varies. At present, BHP is indirectly estimated onsite from the amount of oil and gas production while drilling. For example, if oil and gas production is too high, BHP is probably too low and the negative pressure is too high; on the contrary, if oil and gas production is too low, BHP is probably too higher, which may result in overbalanced drilling. Experiences have proven that manual adjustment of throttle valve to change casing pressure (CP) can indirectly regulate BHP and keep casing pressure within a proper range. However, as manual adjustment has the problems of low accuracy and efficiency, and especially this method of estimating the BHP and adjusting the casing pressure depends on the experiences and competence of the operator in a high degree, and no objective parameters can be directly referred. Any minor mistake in operation may result in overbalanced pressure at hole bottom, which may miss the point of underbalance drilling or even trigger drilling accident in case that the negative pressure is too high.
On the basis of the theory of manual UBD pressure adjustment, Chinese Pat. No. 01136291.X discloses a choke pressure (casing pressure) automatic control system for UBD. It is characterized by collecting dynamic modeling signals (standpipe pressure, casing pressure, etc.) and converting the signals into pressure data by computer, then controlling the pressure following the set casing pressure and standpipe pressure in order to maintain the casing pressure within the set pressure range. Although the accuracy and efficiency are improved when comparing with manual adjustment, the essence of the system is simply to replace manual work with computer, the basic theory and the parameters for reference and adjustment are basically the same as manual adjustment, therefore the same problems with manual adjustment still remain.
At present, the technology for manufacturing rotary blowout preventer specially used for UBD manufacturing tends to mature globally and several Chinese petroleum mechanical factories are also developing rotary blowout preventers, but none of them are equipped with corresponding pressure automatic control system. Furthermore, because of the differences in geology and terrain, the UBD operations conducted abroad usually involve injection of gas, that is, gas and drilling fluid (mud) are injected into drilling tools simultaneously. In UBD with gas injection, BHP is regulated by adjusting the amount of injected gas and injected fluid. Domestic UBD operations, however, are mostly UBD with a single liquid phase, i.e., only drilling fluid is injected into drilling tools. Therefore, the pressure control method used by foreign countries in UBD with gas injection cannot be mechanically applied in China.